1. Field of the Invention
The present invention relates to an improved power roller bearing for use in a toroidal type continuously variable transmission system which is used, for example, as a transmission system of an automobile.
2. Description of the Related Prior Art
In a conventional toroidal type continuously variable transmission system used as a transmission system of an automobile, the rotation of an input disk is transmitted to an output disk in a continuously variable manner through a plurality of power rollers which are respectively included in their corresponding power roller bearings and swingably interposed between the input and output disks.
Here, the power roller bearing comprises: an inner race including a power roller which has a traction portion to be contacted with the above-mentioned input and output disks and also which, due to its rotational movement, can transmit the rotation of the input disk to the output disk; an outer race disposed opposed to the inner race so as to hold the power roller in a freely rotatable manner; and, a rolling ball body which is interposed between the inner and outer races in such a manner as to be held by and between ring-shaped race grooves respectively formed in the mutually opposing surfaces of the inner and outer races, and also which not only transmits a thrust load, which is applied to the inner race from the power roller, to the outer race, but also can be rolled on the race grooves to thereby reduce resistance produced between the inner and outer races during the relative rotation thereof.
As described above, the structure of the power roller bearing, except for the power roller which is provided in the inner race, is almost similar in appearance to that of a thrust ball bearing used to bear a rotary shaft to which a thrust load is to be applied.
In view of this, persons skilled in the art have studied a method for producing the power roller for use in a toroidal type continuously variable transmission system at low costs by diverting parts, which are designed for an existing thrust ball bearing, to the power roller bearing.
However, although the power roller bearing is quite similar in the appearance of the component parts thereof to the thrust ball bearing, the function of the inner race of the power roller bearing is entirely different from that of an ordinary thrust ball bearing. Because of such functional difference of the inner race, the distribution of loads acting on the inner race itself, contact behaviors between the rolling ball bodies, which are interposed between the inner and outer races, and the inner and outer races, and the like are greatly different from those of the ordinary thrust ball bearing. Therefore, in the above component diverting method, there are still left various points to be improved with these differences taken into consideration.
For example, an inner race used in the ordinary thrust bearing serves as a support member for supporting a shaft, whereas a power roller, which is employed in the power roller bearing and can be rotated integrally with its associated inner race, serves as a power transmission member for transmitting the rotation of the input disk to an output disk, that is, it corresponds to a speed change gear in a multistage transmission system of a gear type. And, since such a power roller is rotated at high speeds while it receives a strong pressure from the input and output disks, it generates a great amount of heat; and, such heat generated by the power roller in turn heats the inner race and rolling ball body.
For this reason, as a lubrication oil to be supplied between the inner and outer races, it is indispensable to use a high-viscosity traction oil which has been developed exclusively for the purpose of power transmission.
Also, the traction portion of the power roller to be contacted with the input and output disks provided mutually opposing positions which are located on the outer peripheral edge of the power roller and are spaced 180xc2x0 apart from each other; and, the strong pressures given from the input and output disks are concentrated onto these mutually opposing positions (of the traction portion) as a total force of thrust and radial loads. Therefore, in the traction portion of the power roller to be contacted with the input and output disks, there is generated a very high contact surface pressure.
For example, an ordinary bearing is used at a contact surface pressure of 2 to 3 Gpa or less. On the other hand, in the case of a power roller bearing used in a toroidal type continuously variable transmission system for a vehicle, at a normal decelerating time, the contact surface pressure thereof provides 2.5 to 3.5 Gpa and, at the maximum decelerating time, the contact surface pressure thereof can sometime reach even 4 Gpa.
Further, the strong pressures given from the input and output disks are concentrated onto the 180xc2x0-spaced-apart mutually opposing positions of the traction portion of the power roller as the radial loads, thereby causing the power roller and the inner race, in which the power roller is provided, to be compressed and deformed in the radial direction thereof. Such compression and deformation in turn causes the inner race to be curved. This makes it almost impossible that the thrust loads applied to the inner race from the power roller can be shared uniformly by a plurality of rolling ball bodies respectively interposed between the inner and outer races. That is, the thrust loads to which the rolling ball bodies are subjected become larger on parts of the rolling ball bodies that are situated at positions apart from the above-mentioned mutually opposing positions of the traction portion of the power roller by 90xc2x0. As a result of this, the contact surface pressures of the rolling ball bodies with respect to the race grooves are caused to vary, while part of the rolling ball bodies are caused to roll on the race grooves with a very high contact pressure.
Therefore, the traction portion of the power roller to be contacted with the input and output disks as well as the race grooves of the inner and outer races to be contacted by the rolling ball bodies must be specially adjusted in the material thereof, the hardness of the surfaces thereof, and the surface roughness thereof, in order to prevent the lives thereof from being shortened due to the localized application of the high contact surface pressures.
In view of the above-mentioned background, the present applicants have proposed a technology in which the rolling ball bodies are respectively formed of medium or high carbon steel and the hardness and strength of the surfaces of the rolling ball bodies are adjusted by a carbonitriding treatment or by a quenching and tempering treatment, in order to enhance the durability of the rolling ball bodies against the localized application of the contact surface pressures thereto to thereby be able to improve the life of the bearing (see Japanese Patent Unexamined Publication No. Hei. 7-208568).
Also, the present applicants have further proposed a technology in which input and output disks as well as a power roller and an inner race to be contacted with the input and output disks are carburized and thereafter finished by grinding to thereby adjust the hardness of the surfaces of these components and the effective hardened layer depth thereof to a proper value (in the range of 2 mm to 4 mm) which allows the components to resist the localized application of the contact surface pressures (see Japanese Patent Unexamined Publication No. Hei. 7-71555).
However, the above-mentioned employment of the exclusive traction oil as the lubrication oil to be supplied between the inner and outer races, and the special proper adjustments of the hardness, effective hardened layer depth, and surface roughness of the power roller, inner race and rolling ball bodies through the selection of material and surface treatment, as such, are not sufficient.
In other words, since the original object of the power roller bearing is power transmission, it is important that a dynamic torque loss within the power roller bearing is reduced as much as possible to thereby enhance the transmission efficiency of the torque. However, with employment of only the abovementioned improvements, depending on the dimension setting of the race grooves of the inner and outer races as well as the rolling ball bodies, the dynamic torque loss within the power roller bearing can increase to thereby reduce the torque transmission efficiency.
Also, even if the above-mentioned special proper adjustments of the hardness and effective hardened layer depth of the power roller and inner race have been made, in some cases, there can still arise a problem that the life of the power roller bearing is shortened due to the early breakage of the edges of the race grooves and rolling ball bodies or due to the damage of the contact surfaces of the race grooves and rolling ball bodies.
To solve the above-mentioned problems, the present applicants have experimented and analyzed various design data on the components of the power roller bearing to find the correlation between the design data and the increase or decrease in the dynamic torque loss and bearing life. And, as a result of our elaborate study, it has been found that the radii of curvature of the arc-shaped sections of the race grooves formed on the inner and outer races of the power roller bearing are very closely connected with the increase or decrease in the dynamic torque loss as well as the life of the power roller bearing.
The present invention aims at eliminating the drawbacks found in the above-mentioned conventional power roller bearing and toroidal type continuously variable transmission system. Accordingly, it is a first object of the invention to provide a power roller bearing for use in a toroidal type continuously variable transmission system which can restrict an increase in a dynamic torque loss and also can restrict the shortening of the life of the roller bearing. It is also a second object of the invention to provide a toroidal type continuously variable transmission system which can minimize a dynamic torque loss and improve a torque transmission efficiency due to employment of the above-mentioned power roller bearing.
In attaining the above objects, according to the invention, there is provided, for use in a toroidal type continuously variable transmission system, a power roller bearing for supporting a power roller in a freely rotatable manner, the power roller being swingably interposed between input and output disks, the present power roller bearing comprising: an inner race with the above-mentioned power roller mounted thereon, the power roller having a traction portion to be contacted with the above-mentioned input and output disks and, due to the rotational movement thereof, capable of transmitting the rotation of the input disk to the output disk; an outer race disposed opposed to the inner race for supporting the power roller in a rotatable manner; and, a rolling ball body interposed between the inner and outer races in such a manner that it is held by and between ring-shaped race grooves respectively formed on the mutually opposing surfaces of the inner and outer races, the rolling ball body being capable of transmitting a thrust load, which is applied to the inner race from the power roller, to the outer race and also, due to the rolling motion thereof on the race grooves, reducing resistance produced between the inner and outer races when the inner and outer races are rotated with respect to each other, wherein each of the race grooves respectively formed on the inner and outer races has a cross section of an arc shape, and the radii of curvature of the arc-shaped cross sections of the race grooves are set in the range of 52% to 59% of the diameter of the rolling ball body.
In the power roller bearing which uses a traction oil as a lubrication oil, the dynamic torque loss and bearing life are closely related to the radii of curvature of the arc-shaped sections of the race grooves respectively formed on the inner and outer races of the power roller bearing. In particular, as the radii of curvature of the arc-shaped sections of the race grooves on the inner and outer races increase, the dynamic torque loss decreases. In more particular, when the radii of curvature of the arc-shaped sections of the race grooves on the inner and outer races are set equal to or larger than 52% of the diameter of the rolling ball body, the variation xcex94Ts of the dynamic torque loss is considerably gentle with respect to the slight variation xcex94r of the radius of curvature, and also it is possible to prevent the early breakage of the edges of the race grooves as well as the rolling ball body that is caused by the contact ellipse of the rolling ball body running up onto the edges of the rage grooves.
Also, in the case of the bearing life, as the radii of curvature of the arc-shaped sections of the race grooves on the inner and outer races increase, the bearing life shows a tendency to lower; and, in particular, if the radius of curvature of the arc-shaped section of the race groove on the inner race exceeds 59% of the diameter of the rolling ball body, then the contact ellipse of the rolling ball body becomes excessively small to thereby cause the contact surface of the rolling ball body to be damaged or worn easily, which makes it difficult to secure the basic rated life of the power roller bearing.
In view of the above, if the radius of curvature of the arc-shaped section of the race groove on the inner race is set in the range of 52% to 59% of the diameter of the rolling ball body, then, in the power roller bearing, an increase in the dynamic torque loss can be restricted and also the lowering of the bearing life can be restricted.
Preferably, the inner race may be formed integrally with the power roller.
With use of such integral structure, when compared with the structure in which the inner race and power roller are respectively formed as separate parts, the number of the component parts of the power roller bearing can be reduced. Also, while the power roller is required to be strong since a large load is applied thereto for torque transmission, the inner race can be utilized as an increased diameter portion thereof for increasing the strength thereof, which makes it easy to secure the strength thereof.
Further, even if the power roller is repeatedly deformed due to compression loads applied thereto from the input and output disks, there is no fear that creeping or fretting wear can be caused between the inner race and power roller.
Also, preferably, the radii of curvature of the arcshaped sections of the race grooves on the inner and outer races may be set in the range of 54% to 59% (that is, equal or more than 54% and equal or less than 59%) of the diameter of the rolling ball body.
If the radius of curvature of the arc-shaped section of the race groove on the inner race becomes equal to or more than 54% of the diameter of the rolling ball body, then the inclination angle of the variation xcex94 Ts of the dynamic torque loss with respect to the slight variation xcex94 r of the radius of curvature becomes greatly gentler when compared with a case in which it is set in the range between a value equal or more than 52% and a value less than 54% of the diameter of the rolling ball body.
Therefore, if the radii of curvature of the arc-shaped sections of the race grooves on the inner and outer races are set in the range equal or more than 54% and equal or less than 59% of the diameter of the rolling ball body, then the increase in the dynamic torque loss can be restricted further when compared with the case in which it is set in the range of 52% to 59% of the diameter of the rolling ball body.
Also, in attaining the second object of the invention, according to the invention, there is provided a toroidal type continuously variable transmission system for transmitting the rotation of an input disk to an output disk in a continuous and variable speed manner through a plurality of power rollers, which are respectively included in the above-mentioned power roller bearings and swingably mounted between the input and output disks, wherein the toroidal type continuously variable transmission system uses, as the power roller bearings thereof, the power roller bearing.
According to the above structure, since the power roller bearing used therein can restrict an increase in the dynamic torque loss and also can restrict the lowering of the bearing life, the present toroidal type continuously variable transmission system can reduce the dynamic torque loss in the power transmission from the input disk to the output disk, thereby being able to improve the efficiency of the torque transmission.